1. Field of the Invention
Exemplary aspects of the present invention generally relate to a fixing device and an image forming apparatus including the same, and more particularly, to a fixing device that fixes an unfixed toner image on a recording medium by applying heat and pressure thereto and an image forming apparatus including the fixing device.
2. Description of the Background Art
Typically, a fixing device for fixing an unfixed toner image onto a recording medium is equipped with a roller-type fixing member (hereinafter referred to as fixing roller), a roller-type pressure member (hereinafter referred to as pressure roller), and a halogen heater serving as a heating member. The fixing roller is generally hollow and accommodates the halogen heater inside thereof. Supplying electric power to the halogen heater heats the fixing roller. The pressure roller is pressed against the fixing roller by an urging member.
Such a fixing device is provided with a temperature detector to detect the temperature of the surface of the fixing roller. An output of the temperature detector is provided to a temperature controller that controls the halogen heater based on the output of the temperature detector, thereby maintaining the surface temperature of the fixing roller at a certain temperature.
The fixing device as described above fixes an unfixed toner image on a recording medium by applying heat and pressure to the recording medium as it passes between the fixing roller and the pressure roller, specifically, where the fixing roller and the pressure roller meet and press against each other, hereinafter called a fixing nip or simply nip.
One example of such a fixing device employed in image forming apparatuses such as copiers and printers is a so-called on-demand fixing device. A rise time of this type of fixing device is known to be short.
Various types of on-demand fixing devices have been proposed. One example of a known on-demand fixing device employs a tubular fixing film (endless film) serving as a fixing member, a pressure roller serving as a pressing member, and a heater such as a ceramic heater serving as a heating member. The heater is provided inside the tubular fixing film and pressed against an interior surface of the fixing film, thereby heating the fixing film. The fixing film contacts the pressure roller to form the nip where pressure and heat are applied to the unfixed toner image on the recording medium to fix the unfixed toner image onto the recording medium.
In order to prolong the product life of the fixing member and prevent paper jams and fixing defects such as “hot offset” in this type of fixing device, the fixing member, that is, the tubular fixing film, needs to be heated evenly and stably. “Hot-offset” herein refers to an undesirable phenomenon in which part of a fused toner image is adhered to the surface of the heating member, and is re-transferred onto the sheet itself or the following sheet of the recording material.
In view of the above, several approaches have been tried to reliably achieve a proper fixing operation. For example, in order to prevent hot-offset, Japanese Patent Application Publication No. (hereinafter “JP-A”) 2002-311749 proposes to reduce a fixing temperature in accordance with a number of prints when printing in a low speed mode, and includes a temperature estimation mechanism that estimates the temperature of the pressure roller when printing in a high speed mode. In this configuration, the fixing temperature is determined according to the result and the number of prints.
By contrast, JP-2001-201978-A discloses a method of controlling the temperature of the fixing belt. Specifically, the heating roller of the fixing device includes a plurality of heat sources, that is, a first heat source having a first luminous intensity distribution (thermal distribution) substantially corresponding to the area of a sheet of paper of small size and a second heat source having a second luminous intensity distribution substantially corresponding to both ends of the heating roller that is not in the first luminous intensity distribution. Further, the fixing belt includes first and second temperature detectors for detecting temperature in the first and the second luminous intensity distributions, respectively.
In this configuration, the temperature of the fixing belt is adjusted to a specified temperature based on the temperature detected by the first and the second temperature detectors. Disadvantageously, however, using two heat sources increases the size of the fixing device and its cost, thereby complicating efforts to make the image forming apparatus as a whole as compact and low cost as is usually desired.
Although generally advantageous, such related-art on-demand fixing devices using the tubular fixing roller have a drawback in that the range of shapes of the nip is limited, which restricts ways to improve fixing ability as well as prevent curling and creasing in different kinds of recording media.
Furthermore, typically, the on-demand fixing devices employ the film-type fixing member to achieve short rise time. In order to achieve that effect, the film-type fixing member needs to be heated evenly and stably. However, in general, the fixing member is subjected to highly localized heating by the heating member. Thus, if the fixing member remains stationary and does not move, such that it is heated continuously by the heating member, the temperature of that particular portion of the fixing member being heated rises significantly, causing deformation of and damage to the fixing member.
Accordingly, in an effort to monitor the operating state of the fixing member, that is, to detect movement or rotation of the fixing member, conventionally the movement of the fixing member is detected by detecting an operation of a drive source of the fixing member or a driving force transmission device. However, in this approach, even when the driving force is not properly transmitted to the fixing member from the drive source, for example, when the fixing member slips out of rotation with the drive source, leaving the fixing member immobilized even as the driving force continues to be transmitted to the fixing member, the fixing member is erroneously detected as being rotated.
In view of the above, a small-size, low-cost, on-demand fixing device that reliably fixes an unfixed toner image for an extended period of time is required.